1. Field of the Invention
The invention is related to the field of Nuclear Magnetic Resonance (xe2x80x9cNMRxe2x80x9d) apparatus and methods. More specifically, the invention relates to detecting and estimating the effect of transversal motion of the NMR tool used in oil well logging on the signal-to noise ratio by using both in-phase and out-phase measurements of spin echoes.
2. Description of the Related Art
NMR has applications in various fields from medical applications to oil well logging applications. In oil well testing, NMR is used to determine, among other things, the porosity of the material, the amount of bound liquid in the volume, permeability, and formation type, as well as oil content.
A current technique in wellbore logging employs an NMR tool to gather information during the drilling process. This technique is known as logging while drilling (LWD) or measuring-while-drilling (MWD) and requires the NMR tool to be included as part of the drilling bottom hole assembly. This process greatly increases speed at which information is gathered and consequently reduces the cost of acquiring downhole information. This tool can be, as an example, one that is outlined in U.S. Pat. No. 5,280,243, entitled, xe2x80x9cSystem For Logging a Well during the Drilling Thereofxe2x80x9d, granted to Miller. The device disclosed therein includes a permanent magnet which induces a static magnetic field into the surrounding volume. In addition, an antenna, which is aligned orthogonal to this magnet, has the purpose of introducing radio frequency (RF) pulses into the region. The same or another antenna is used to receive signals returning from the volume.
Typically, in the presence of only the permanent magnet, nuclear spins will align either parallel or anti-parallel to the static magnetic field, creating a net overall magnetic polarization, called a bulk magnetization. An electric RF pulse sent through this antenna induces another magnetic field in the region. If this induced magnetic field is perpendicular to the field of the permanent magnet, then the induced magnetic field pulse reorients the direction of individual spins perpendicular to the direction of the static field and to the direction of the induced magnetic field. Upon removing the RF pulse, the spins will relax by realigning to their original orientation, along the axis of the static field. The relaxation of the spins to their original orientation occurs over a characteristic time interval, which is known as the spin-lattice relaxation time, T1. This relaxation induces a voltage in the receiver antenna.
Spins oriented perpendicular to the static field undergo other motions which can be measured. The spin vector relaxes out of this transverse direction with a characteristic time known as the spin-spin relaxation time or transverse relaxation time, T2. Typically, a pattern of RF pulses can be used to determine T2. A commonly used pulse pattern is known as the Carr-Purcell-Meiboom-Gill (CPMG) sequence. The CPMG is comprised of one pulsed magnetic field applied in a direction orthogonal to the static magnetic field followed by several pulses applied at preset time intervals in a direction mutually perpendicular to both the direction of the first pulse and the direction of the static magnetic field. The first pulse of the CPMG sequence is known as the A-pulse, and typically occurs over a short time scale with respect to the relaxation time, T2. In response to the A-pulse, the spin vectors of the nuclei will align along a common direction in the plane that is perpendicular to the static magnetic field. When an individual spin vector is placed perpendicular to an applied external field, it will precess around the field with a frequency of precession known as the Larmor frequency, which is related to the strength of the applied field. Due to inhomogeneities in the magnetic field, some spins will precess faster while other spins will precess more slowly. Thus, after a time long compared to the precession period, and short compared to T1, the spins will no longer be precessing in phase. The diffusion of the phase of the precession takes place over a time scale T2*. For an acceptable observation, it is best to have T2 greater than  greater than T2*.
The B-pulse of the CPMG sequence lasts twice the duration of the A-pulse and is also short compared to precession periods and to relaxation time. Applying the B-pulse gives the nuclear spins an axial rotation of 180 degrees from their immediately previous orientation. In the new orientation after applying the B-pulse, the spins, which were previously diverging from their common orientation due to the A-pulse, are now returning towards this orientation. In addition, by inverting the spatial relation of leading and lagging precessors, the spins are now moving back into phase. As the spins realign, the cumulative effect of this alignment causes a spin echo. The sudden magnetic pulse of the spin echo induces a voltage in the receiving antenna.
Once the spins have realigned and produced the spin echo, they will naturally lose phase again. Applying another B-pulse flips the spin orientation another 180 degrees and sets up the condition for another spin echo. By a using a train of B-pulses, the CPMG pulse pattern creates a series of spin echoes. The amplitude of the train of spin echoes decreases according to the relaxation time, T2. Knowledge of T1 and T2 gives necessary information on the properties of the material being examined.
Measurements made for T1 and T2 require that the NMR measuring device remain stationary over the proper time period. However, a typical measurement period can last over 300 msec. Over a testing period that is sufficiently long, the measuring device will be susceptible to motion from its initial position. At the beginning of the testing period, the permanent magnet might polarize spins of nuclei remaining within a given volume, which can be seen in FIG. 6 as the shaded volume 20a. It is necessary for a certain amount of time to lapse for these spins to polarize completely. If the NMR tool moves during this time, the volume 20a changes its position as shown in FIG. 7. At this new position, the volume 20a contains only a portion of the original volume shown in FIG. 6, and the receiving antenna will necessarily record unsaturated spins from the new volume. Instead, the new volume contains spins that are not properly aligned to the static field. This effect is typically referred to as xe2x80x9cmoving fresh spins inxe2x80x9d and is a source of error in the detection signal. As an example, the measurement may yield a bound fluid volume (BFV) that is higher than the amount that is actually present in the region.
Several methods have been proposed to detect motion in order to address the problems this motion introduces. Among these methods include use of strain gauges, an ultrasonic range finder an accelerometer, or a magnetometer. These arrangements are described in PCT Application Number PCT/US97/23975, titled xe2x80x9cMethod for Formation Evaluation While Drillingxe2x80x9d filed Dec. 29, 1997. These motion detection devices help to set a threshold to establish the quality of the recorded data. However, they do not provide a means to make corrections which might maintain the quality of the data.
Another proposed device is detailed in European Patent Application 99401939.6, titled xe2x80x9cDetecting tool motion effects on nuclear magnetic resonance measurements.xe2x80x9d This application uses different geometries and magnetic gradients to measure tool motion. Given the same motion rates of the NMR tool, the signals from two regions of differing applied magnetic gradients will decay at different rates. In the application, setting up an apparatus with two magnetic field gradients makes it possible to obtain both signals and thereby determine the motion speeds and the necessary corrections. Similar information can be derived by measuring spin-echoes in two radially-adjacent regions.
Different magnetic field gradients are easily achieved by placing several permanent magnets in various spatial arrangements with respect to one another. For example, shortening the distance between the north poles of magnets can increasing the magnetic field gradient. NMR signals received from regions with higher magnetic field gradients are more sensitive to motion than those received from regions with lower magnetic field gradients. Specifically, when the NMR tool is in motion, a signal received from a high gradient region decays at a rate more slowly than a signal coming from a low gradient region. Comparing the relative decay rates of signal strengths from each region allows a determination of the amount of motion of the NMR tool. Erroneous calculations may be introduced, since the low gradient region and the high gradient region are separate volumes.
Another method that has been taught is to truncate the pulse sequence to the order of 10 milliseconds rather than 300 msec. This procedure is taught in U.S. Pat. No. 5,705,927 issued to Kleinberg. At such short times, the quality of the data remains acceptable. However, not always will there be enough data to extrapolate values for T2.
There is a need for a method of determining from the NMR signals themselves indications of when the data quality is likely to be acceptable. The present invention satisfies this need.
The present invention is a method of making Nuclear Magnetic Resonance (NMR) measurements. A magnet on an NMR tool is used to generate a static magnetic field in a volume containing materials sought to be analyzed. A radio frequency (RF) transmitter antenna on the NMR tool induces a RF magnetic field in the volume and excites nuclear spins of nuclei therein, the RF magnetic field being substantially orthogonal to the static field in said volume. When the tool is subject to transversal motion, the spin-echo signals are affected by the tool motion. A receiver antenna is used for receiving in-phase and quadrature components of signals from said excited nuclei. A phase drift indicator may be determined from the in-phase and quadrature components of said signals. This phase drift indicator is diagnostic of tool motion.
The method of the present invention may be used with any of a number of different types of logging tools having different magnet and coil configurations. These include tools with opposed magnets, and transverse dipole magnets.
The method of the present invention may be used with conventional CPMG sequences or with modified sequences designed for reduced power consumption having B pulses that are less than 180xc2x0. Phase alternated pairs of measurements may be used to reduce the effects of ringing.
The phase drift indicator is preferably determined as the ratio of a windowed sum of the magnitudes of the quadrature component signals to the windowed sum of the magnitudes of the in-phase component signals.